Dynamical parameters associate with medium range oscillations of summer monsoon rainfall over India

Abstract

Mean circulation patterns antecedent and simultaneous with spells of excess and deficient weekly rainfall over the major parts of the subcontinent have been studied. Strong southerly flow in the mean circulation occurs in the lower troposphere during spel1s of scanty or deficient rainfall, over most parts of the country with opposite type of meridional circulation during active spells. In the upper troposphere (300 mb) westerly flow regime protrudes southwards and extends almost along 25 deg. N during spells of scanty rainfall while during spells of excess or normal rainfall easterly regime prevails up to 30 deg. N over the country. The divergence and vorticity patterns indicate areas of divergence over extreme northeast and northwest India during deficient and scanty rainfall regime in the lower troposphere and areas of convergence over northwest India and east coast in the upper troposphere. The dynamical parameters and their utilisation for medium range forecasting of a drought in the summer monsoon season are discussed.